1. Field of the Invention
The present invention relates to a case hardening steel superior in property of preventing coarse grains at the time of carburizing and in fatigue property, and a manufacturing method thereof.
Priority is claimed on Japanese Patent Application No. 2009-008174, filed on Jan. 16, 2009, the contents of which are incorporated herein by reference.
2. Description of Related Art
For gears, bearing components, rolling components, shafts, and constant velocity joints, a medium carbon alloy steel for machine structural purposes, as defined, for example, in JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, or the like in Japanese Industrial Standards (JIS), is typically used. These components are typically manufactured through steps of being worked into a predetermined shape by cold forging (including rolling) or hot forging, and cutting, and then carburizing and quenching. In cold forging, products have a good surface and dimensional accuracy, and manufacturing costs are lower than that in hot forging, and yielding is favorable. Consequently, there is an increasing tendency to use cold forging to manufacture components that have been conventionally manufactured by hot forging. As a result, carburized components manufactured in steps of cold forging and carburizing has obtained significantly wider applications and uses in these years. An important challenge in the manufacture of carburized components is how to decrease distortion in heat treatment. A solution to this problem is required for example, in the following cases. In the case of applying a carburized component to a shaft, if a bending deformation happened to the shaft by heat treatment distortion, the function of component as a shaft is impaired. Furthermore, in the case of applying a carburized component to a gear or a constant velocity joint component, an increase in heat treatment distortion leads to a cause of noise or vibration. Here, the greatest cause of heat treatment distortion produced in a carburized component is coarse grains produced at the time of carburizing. To suppress coarse grains produced at the time of carburizing, annealing is conventionally performed after cold forging, before carburizing and quenching. However, particularly in recent years, there is an increasing tendency toward omission of annealing in terms of cost reduction. Therefore, there is a strong demand for a steel material that will not produce coarse grains in a carburized component even in the case where annealing is omitted.
On the other hand, among the gears, bearing components, and rolling components, the bearing components and rolling components on which a high surface pressure is loaded are subjected to a deep carburizing. Deep carburizing requires a long period of time, typically ten and some hours to tens of hours. Therefore, in terms of reducing energy consumption, shortening of the carburizing time is an important challenge. For shortening the carburizing time, increase in carburizing temperature and increase in carbon amount of the base material before carburizing are effective. The typical carburizing temperature is approximately 930° C. If a so-called high-temperature carburizing is performed in a temperature range of 990 to 1090° C., there arises a problem in that coarse grains are produced, and it becomes unable to obtain required fatigue properties, rolling contact fatigue properties, and the like. Therefore, there is a demand for a case hardening steel which does not produce coarse grains even in high-temperature carburizing, that is, a case hardening steel suitable for high-temperature carburizing. For example, to obtain the same effective case depth as that at the time of regular carburizing, the carburizing time is expected to be shortened by using the base material having a higher carbon content before carburizing, i.e., 0.2% to 0.3% of C, as for the level of regular hardening steel level.
In particular, many of the gears, bearing components, and rolling components on which a high surface pressure is loaded are offered as large-size components, and are typically manufactured through the steps of “steel bar, hot forging, heat treatment such as normalizing, if required, cutting, carburizing and quenching, and polishing if required.” To suppress production of coarse grains at the time of carburizing, it is necessary to achieve a material quality suitable for suppressing coarse grains in a state after hot forging, that is, in a state of a hot forged member. To do so, it is required to achieve a material quality suitable for suppressing coarse grains in a state of a base material for a steel bar or wire rod.
As a technique for stably suppressing coarse grains in conventional case hardening steel, there is disclosed a case hardening steel that includes predetermined amounts of Al and N and is excellent in property of preventing coarse grains in which the state of a ferrite band of a structure in a cross-section parallel to the hot rolling direction is appropriately regulated (for example, see Japanese Unexamined Patent Application, First Publication No. H11-106866, hereinafter Patent Document 1). However, as for the disclosed technique in Patent Document 1, there are actually cases where components manufactured through the steps of spheroidizing annealing and cold forging are not stably exerting effects of suppressing coarse grains. Furthermore, also in high-temperature carburizing, there are actually cases where it is not able to suppress production of coarse grains.
Furthermore, Japanese Unexamined Patent Application, First Publication No. H11-92863 (hereinafter Patent Document 2) discloses a manufacturing method of a case hardening steel in which a steel material including, by mass %, Ti: 0.10 to 0.30% and N: less than 0.01% in addition to predetermined amounts of C, Si, or the like is used, and in which hot rolling heating for a slab is performed within a temperature range of 1250 to 1400° C. and also rolling heating for a product is performed at a temperature of Ac3 to 1050° C. Furthermore, Japanese Unexamined Patent Application, First Publication No. H11-92824 (hereinafter Patent Document 3) discloses a technique of improving a rolling contact fatigue life and rotating bending fatigue life by finely dispersing Ti carbides in a case hardening steel having similar compositions to those of Patent Document 2.
Furthermore, in Japanese Unexamined Patent Application, First Publication No. 2003-34843 (hereinafter Patent Document 4), there is disclosed a high-strength case hardening steel that includes, by mass %, Ti: over 0.1 to 0.2% and N: 0.015% or less in addition to predetermined amounts of C, Si, or the like and is made of a martensitic structure in which a prior austenite grain size is refined to standard No. 11 of JIS G0551 or further. Furthermore, there is disclosed a high-strength case hardening steel that includes, by mass %, N: 0.020% or less and includes one or more of “Ti: 0.05 to 0.2%, V: 0.02 to 0.10%, and Nb: 0.02 to 0.1%,” and is made of a martensitic structure in which a prior austenite grain size is refined as No. 11 of JIS G0551 or further.
Furthermore, Japanese Unexamined Patent Application, First Publication No. 2005-240175 (hereinafter Patent Document 5) discloses a case hardening steel that is excellent in property of preventing coarse grains during carburizing and in fatigue property, in which the case hardening steel includes, by mass %, Ti: 0.05 to 0.2% and a specific range of another specific component, and N is limited to less than 0.0051% by mass %; or in which the case hardening steel further includes Nb: less than 0.04% by mass % and an amount of precipitates of AlN after hot rolling is limited to 0.01% or less, or in which a structural fraction of bainite after hot rolling is further limited to 30% or less, or in which the ferrite grain size index after hot rolling is further limited to any of No. 8 to No. 11 as defined in JIS G0552, or in which a maximum diameter of Ti-based precipitates of a matrix of a steel after hot rolling in a cross-section in a longitudinal direction is further limited to 40 μm or less, the maximum diameter being obtained by extreme value statistics measured under the following conditions.
However, the coarse grain preventive steel into which an abundance of Ti is doped as shown in the aforementioned Patent Documents 1 to 5 have a problem in that the abundant doping of Ti degrades machinability. Major causes of the degradation in machinability are: 1) an increase in hardness of the base material and 2) a decrease in MnS that has a strong effect of improving machinability, due to production of Ti-based nitrides. With these causes, for example a tool used for working on steel is likely to be damaged, resulting in a problem of machining being made less easy, or the like, that is, the machinability is degraded.
Furthermore, as for methods of shortening the time for carburizing, there exits an increase in carburizing temperature and also an increase in carbon content in the base material before carburizing. However, n this latter case, degradation in machinability resulting from an increase in hardness of the base material is a problem.